The correct answer is option D.
Noting the color of a solution when heated is an example of an experimental scientific investigation.
Option A, noting the PH of a solution on a digital meter is an example of experimental data recording.
Similarly, option B recording the sound of glass breaking, is an example of experimental record keeping.
Option C, placing test tubes in an ice bath for 15 min is an example of experimental procedure.
But option D, noting the color of a solution when heated is an example of experimental scientific investigation. Because the scientist is make an acute observation of the color change whenever the solution is heated.